Doctors conducted a pilot trial after identifying which immune cells were responsible for destroying hair follicles in people with the condition.

Within four or five months of being put on the drug, ruxolitinib, all three patients experienced complete hair regrowth.

"We've only just begun testing the drug in patients, but if the drug continues to be successful and safe, it will have a dramatic positive impact on the lives of people with this disease," said Dr Raphael Clynes, from Columbia University Medical Centre in New York.

However, the team said further tests were needed before the drug can safely be used as a baldness treatment.

Alopecia is a common autoimmune disease that can lead to total hair loss.

Joanna Rowsell, the Olympic gold medal cyclist, has suffered from the condition for 13 years. The 23-year-old removed her helmet at the London 2012 Games to reveal that she is almost totally bald and collected her medal in front of an audience of 17 million people, without her usual wig.

The model and television presenter Gail Porter has refused to wear a hat or wig to hide the condition, which she has suffered from since 2005.

There is no connection between alopecia and male-pattern baldness, the hormone-driven hair loss that affects 6.5 million men in the UK.

The trial followed tests on mice using two new drugs known as JAK inhibitors that can be taken in pill form and are used to block immune pathways.

Ruxolitinib is approved for the treatment of a form of bone marrow cancer in both America and Europe. Another drug, tofacitinib, is licensed for the treatment of rheumatoid arthritis in the US but not Europe.

In experiments on mice, both drugs completely restored the hair of animals with alopecia within 12 weeks.

The trial patients all had moderate-to-severe alopecia areata, which causes patchy loss of head hair. Gail Porter is affected by a more serious form of the condition, called alopecia totalis, which results in complete baldness.

Each patient was given 20mg of ruxolitinib twice a day. The drug's effectiveness was linked to the disappearance of immune cells that attack hair follicles.

"We still need to do more testing to establish that ruxolitinib should be used in alopecia areata, but this is exciting news," Dr Clynes said.

The research appears in the latest online edition of the Nature Medicine journal.